Chemical mechanical polishing (CMP) is presently used to polish a variety of materials found in semiconductor devices. Those materials include metals, such as tungsten, aluminum, and copper. Regardless of the type of material being polished, similar techniques are used. For example, a polishing system typically includes a polishing platen, on which is attached a polishing pad. While the platen is being rotated, a slurry is dispensed while a semiconductor wafer is pressed against the pad. A combination of the chemical reaction between the slurry and the layer being polished and the mechanical interaction between abrasives within the slurry and the layer being polished cause the planarization of the layer.
In some instances, two layers of different materials are deposited on each other in a semiconductor substrate, and both materials need to be polished, preferably in a continuous polishing operation to minimize cycle time. Commercially available polishing slurries do not provide ideal properties for polishing two dissimilar materials during the same polishing operation. For example, when polishing tungsten that is deposited on a titanium/titanium nitride layer, the polishing properties of tungsten and the titanium layer differ greatly. Titanium is a relatively difficult material to polish using a slurry composition optimized for tungsten polishing. Slurry formulations that successfully polish titanium typically do not polish tungsten as fast as other slurries. In most cases, optimizing the polishing conditions for one material, for example tungsten, leads to a degradation of the polishing characteristics of the other materials, such as titanium.
One known method for polishing a combination of tungsten and titanium is to use a relatively hard or abrasive polishing pad, such as a Suba 500 made by Rodel, Inc. of Delaware, with a slurry formulated for tungsten polishing (e.g. a ferric nitrate slurry). The polishing slurry does not significantly chemically react with the titanium, therefore use of a harder polishing pad is effective in mechanically removing titanium. However, problems with this method include 1) a lower tungsten polishing rate than if a softer pad is used; and 2) high oxide removal or erosion during polishing. Oxide removal or erosion is undesirable because it is generally non-uniform across the wafer, being faster in dense feature arrays and slower in peripheral areas. Use of a softer pad, such as a Politex pad, also by Rodel, Inc. of Wilmington Delaware, results in less removal of oxide, but inadequately removes titanium.
Another method for overcoming the problem of polishing tungsten and titanium is to polish the tungsten away, but leave the titanium layer in place. An interconnect metal layer(s), such as aluminum, is then deposited on the remaining titanium layer, and the aluminum and titanium layers are simultaneously patterned and etched. By etching the titanium layer with the aluminum, the need to polish away the titanium is eliminated. However, the titanium layer is nonetheless exposed to the polishing process during the polishing of tungsten. Consequently, the quality of titanium under the aluminum is poor and reliability of the resulting aluminum interconnects is degraded.
Accordingly, there is a need in the industry to establish a polishing process that can effectively polish two dissimilar conductive materials in a cost effective manner that is conducive to a manufacturing environment.